Information Notice No. 92-20: Inadequate Local Leak Rate Testing

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 3, 1992
NRC INFORMATION NOTICE 92-20: INADEQUATE LOCAL LEAK RATE TESTING
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems involving local leak rate testing
(LLRT) of containment penetrations under Part 50 of Title 10 of the
Code of Federal Regulations (10 CFR 50), Appendix J. It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances
Quad Cities Station, Unit 1
On February 26, 1991, the Commmonwealth Edison Company (the licensee)
performed a Type B LLRT on the containment penetration bellows for
penetration X-25 at Quad Cities Station, Unit 1, and found an acceptable
measured leakage rate of 6 standard cubic feet per hour (scfh). The
licensee performed this LLRT by pressurizing the volume between the two
plies of the bellows through a test connection. On February 28, while
performing the primary containment integrated leak rate test (ILRT) under 10
CFR 50, Appendix J, the licensee found excessive air leakage from the
penetration. The licensee recognized the inconsistencies between the LLRT
data for the penetration and the ILRT results and began a test program to
determine the source of the error. Using a blank flange on the containment
side of the bellows, the licensee pressurized the bellows for a "local
ILRT," which yielded a leak rate of 137 scfh. The licensee also repeated
the Type B LLRT with holes drilled in the bellows. This LLRT result (8
scfh) was only slightly higher than the previous LLRT result. The results
of this test program led the licensee to conclude that it is not possible to
perform a valid Type B LLRT on this type of bellows assembly. The licensee
replaced the bellows for penetration X-25 due to cracks identified by the
tests. Commonwealth Edison is also investigating alternative test methods
that would provide accurate LLRT results for bellows penetration assemblies.
This problem was reported to the NRC under Title 10 of the
Code of Federal Regulations, Part 21 (10 CFR 21).
9202260208
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IN 92-20
March 3, 1992
Page 2 of 3
Dresden Nuclear Power Station, Unit 2
On December 17, 1990, the Commonwealth Edison Company (the licensee) found a
leakage rate significantly greater than the maximum allowed during the
pressurization phase of its ILRT. The licensee identified the source of the
leak as the inboard flange of the torus purge exhaust inner isolation valve
with an estimated leakage rate of approximately 25 weight percent per day at
15 psig. The licensee had last performed maintenance on this valve during
the previous outage. Although a LLRT had been performed on the valve
following the maintenance, the test did not challenge the inboard flange.
Perry Nuclear Power Plant, Unit 1
On July 7, 1989, the Cleveland Electric Illuminating Company (the licensee)
discovered a leak through the inboard flange of the seal leakoff line on a
relief valve for the residual heat removal system during the plant's first
periodic ILRT. The licensee had performed maintenance on the valve three
times from 1986 to 1989. In each case, the licensee had performed a LLRT
following the maintenance. However, the LLRT did not challenge the inboard
flange.
Clinton Power Station, Unit 1
On December 18, 1990, the Illinois Power Corporation (the licensee) found
that lines from the residual heat removal system relief valves were not
water-sealed under post-accident conditions as previously indicated in its
safety analysis report. These lines were intended to terminate below the
suppression pool minimum drawdown level, allowing the water to maintain a
seal on the containment isolation valves under accident conditions. The
problem was first identified when it was discovered that a line, considered
to be water-sealed, included a vacuum breaker. The vacuum breaker would
open following an accident, bypassing the water seal. The licensee
investigated this condition and found that a number of other lines that
empty into the suppression pool either contained flanges or terminated above
the pool minimum drawdown level. Since these lines would have been open to
the containment atmosphere following an accident, the associated isolation
valves should have been tested for leakage using Type C air tests. To
correct this problem, the licensee removed the vacuum breaker connections
and the flanges and extended the pipes to ensure that a water seal would be
maintained.
Discussion
Steel expansion bellows are used on piping penetrations in many plants as
part of the containment isolation scheme. A Type B LLRT is performed on the
bellows periodically to verify that containment integrity is being
maintained. The event at Quad Cities revealed that the LLRT performed
between the two plies could not be used to accurately measure the leakage
rate that would occur through the bellows under accident conditions. The
two plies of the bellows were in contact with each other, restricting the
flow of the test medium to the crack locations. The NRC staff investigated
and found that this problem is not isolated to the bellows manufactured by
the vendor involved at Quad Cities. Any two-ply bellows of similar
construction may be susceptible to this problem.
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IN 92-20
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Page 3 of 3
The NRC granted an exemption from the Type B testing requirements of
10 CFR 50, Appendix J, to Commonwealth Edison for Quad Cities and Dresden on
February 6, 1992. The exemption covers the testing of the two-ply bellows
discussed in this information notice because no valid Type B LLRT can be
performed on these bellows. The exemption specifies an alternative program
of bellows testing and replacement that involves testing with air at a
reduced leakage limit, testing any leaking bellows with helium (sniffer
testing), replacing bellows that are unacceptable, and performing an ILRT
each refueling outage until all of the bellows have been replaced with
testable bellows.
The two events involving leaking flanges occurred because the licensees
failed to consider all possible leakage paths when they established their
leak rate test programs. Both licensees identified the valves involved in
the events as containment isolation barriers, but they failed to consider
the gasketed flanges as leakage paths. Both licensees tested the isolation
valves in the reverse direction which did not challenge the flanges
properly. Any containment isolation valve could have this problem,
particularly if the valve is tested in the reverse direction or if both
valves on a penetration are outside of containment.
Appendix J to 10 CFR Part 50 requires all licensees to perform local leak
rate testing on containment isolation valves. The licensees for some
plants, including Clinton, have received credit from the NRC for maintaining
a water seal on the valves instead of performing local leak rate testing
using air as the test medium. One requirement for a valid water seal is
that the penetration have no potential air leakage paths, such as flanges or
vacuum breakers. For those lines that depend on the minimum water level in
the suppression pool to prevent leakage, it is expected that the lines
terminate below the minimum suppression pool level in the actual plant
configuration.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: M. P. Phillips, RIII A. J. Kugler, NRR
(708) 790-5530 (301) 504-2828
F. A. Maura, RIII J. C. Pulsipher, NRR
(708) 790-5696 (301) 504-2811
Attachment: List of Recently Issued NRC Information Notices
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